1. Field of the Invention
This invention relates to infra-red (e.g. thermal) imaging systems and to other optical systems, and in particular to optical systems in which at least two optical parameters can be controlled relative to one another.
2. Discussion of Prior Art
It is known to produce optical systems comprising two or more optical components of fixed focal length in which variation of at least two optical parameters of the system is achieved by moving two or more of the optical components relative to one another.
For example it is known to produce a variable magnification compound lens comprising at least two optical components of fixed focal length wherein the overall optical magnification of the system is varied by adjusting the spacing between the optical components whilst an in-focus image is maintained on a fixed image plane by relatively varying the distance from the final optical component to the fixed image plane.
The technology used in such systems is well developed and provides acceptable results in most applications. However, the mass and response time of these systems can be adversely affected by the need to physically move the optical components. Furthermore, for critical applications, especially in spacecraft optics, moveable optical components require complex counterbalancing arrangements.
It has also been known for nearly ten years from U.S. Pat. No. 4,836,661 to propose a refractive variable magnification system for zoom lenses in which a number of refractive lenses of variable refractive power are provided a fixed distance apart and are used to focus an image onto a fixed image plane.
U.S. Pat. No. 4,890,903, published in 1990, discloses a lens unit having variable focus refractive lenses which can be bodily rotated to alter their focal length, and which are gas or fluid filled. One lens have a positive power and the other a negative power. Uses in spectacles and cameras are disclosed.
U.S. Pat. No. 4,630,903 shows a complex multi-refractive lens system for a photocopies. it requires the ability to vary 3 system parameters from 6 in a refractive system.
It has also been known for many years in the field of thermal imaging to have detector, usually pixellated, upon which an image is focused and provided with a chopper whose function is to cause that image to dither over the detector, and to present a periodic reference signal to the detector. A typical chopper has three segments: a first transmissive prism, a second transmissive prism, and an opaque shutter region, the two prism sections causing an image to fall upon slightly different regions of the detector, and the shutter region providing a black body reference signal which can be used to allow the output voltage of a pixel to fall away from a scene-dependent value.
A paper by N. Butler, J. McClelland and S. Iwasa, employees of Honeywell, entitled xe2x80x9cAmbient Temperature Solid State Pyroelectric I.R. Imaging Arraysxe2x80x9d, dated March 1988, NE 8701-02 (SPIE), discusses using a de-focusing chopper having a thick transparent xe2x80x9cblurringxe2x80x9d portion.
It has also been known to compensate for movement of a detected image relative to a sensor/sensor array by introducing special, and additional, components into the optics of an imaging system, for example to compensate for atmospheric alteration, and camera shake.
According to a first aspect of the present invention there is provided an optical system comprising two or more variable focal length optical components whose positions are fixed relative to one another; the system further comprising control means for varying the focal length of at least one of the said variable focal length optical components in relation to the focal length of at least one of the other said variable focal length optical components such that, in use, control over at least two optical parameters if the system is achieved.
The invention overcomes the drawbacks of much of the prior art because variation of certain optical parameters of the system can be achieved without the need to change the relative positions of the optical components within the system, i.e. without the need to, for example, move the optical components bodily, as a whole, towards or away from each other, or to rotate bodily the optical components. As a result system reprogramming can be achieved in time-scales shorter than those required for physical movement of the positions of the optical components, this being of particular benefit in active optical systems. (By active optical system it is meant an optical system in which the focal lengths of the optical components are varied in real time according to predictions regarding the required focal lengths, as opposed to being controlled by a feedback loop.) Furthermore, if the power supplies required for varying the focal length of the optical components are of low-mass design, then the overall mass of the system may be lower than for a conventional system due to the lack of mechanical positioning gear. Also the need for complex counterbalancing requirements is eliminated for critical applications such as spacecraft optics.
The optics may be used in a beam expander; or in a zoom lens unit; or in a camera; or in binoculars or a telescope. The optics may be reflective. An all-reflective optics device may be provided.
The control means in an optical system according the first aspect of the invention may comprise a mechanical linkage, electronic circuit or a computer program.
An optical system according to the first aspect of the invention may further have an image plane fixed relative to the said variable focal length optical components in the system.
One optical parameter that may be controlled is the magnification of the system. Alternatively, the focal lengths of the first and second components may be the two controlled parameters. Tilt means may be provided to apply a two-axis tilt to one or more of the optical components. De-focusing means may be provided to de-focus the image received by the detector/detector array.
In one application of an optical system according to the first aspect of the invention, variation of the focal length of at least one of the said variable focal length optical components in relation to the focal length of at least one of the other said variable focal length optical components varies the position of a principle plane of the system in dependence on a change in effective focal length of the compound system such that an image of variable magnification is obtained with maintenance of a substantially in-focus image in a fixed image plane.
In a further application of an optical system according to the first aspect of the invention, variation of the focal length of at least one of the said variable focal length optical components in relation to the focal length of at least one of the other said variable focal length optical components may vary the width of an optical beam whilst substantially maintaining collimation of the beam.
According to a second aspect of the present invention there is provided an optical system for varying the width of an optical beam, comprising two or more variable focal length optical components whose positions are fixed relative to one another, and control means for varying the focal length of at least one of the said variable focal length optical components in relation to the focal length of at least one of the other said variable focal length optical components such that, in use, the width of the optical beam is varied whilst substantially maintaining the collimation of the beam.
The width of the optical beam may be expanded, or may be contracted.
The optical components may comprise refractive optical components, or may comprise reflective optical components, such as mirrors.
According to a third aspect of the invention there is provided a variable magnification zoom lens unit comprising two or more variable focal length optical components whose positions are fixed relative to one an other and to an image plane of the lens unit; the lens unit further comprising means for varying the focal length of at least one of the variable focal length optical components relative to the focal length of at least one other of the variable focal length optical components such that, in use, an image of variable magnification can be obtained with maintenance of a substantially in-focus image in the fixed image plane.
According to a fourth aspect of the invention there is provided a method of controlling at least two optical parameters of an optical system having two or more optical components comprising the steps of:
a) fixing the positions of at least two of the optical components relative to one another; and,
b) relatively varying the focal length of at least two of the said fixed position optical components.
According to a fifth aspect of the present invention there is provided an optical system comprising at least first and second variable focal length reflective optical components whose positions are fixed relative to one another; the system further comprising control means for varying the focal length of the first optical component and for varying the focal length of the second optical component, the control means being capable of controlling the first and second variable focal length reflective optical components so as to achieve a change in relative focal length between the first and second components such that, in use, control over at least two optical parameters of the system is achieved.
The optical system preferably operates over a wide spectral range, for example from 450 nm to 10,000 nm. A silvered mirror has a reflectivity of about 0.9 at a wavelength of 450 nm and a reflectivity of 0.99 at a wavelength of 10,000 nm. The optical system has a focal plane which is preferably at the same place for all wavelengths over which the system is designed to operate. This is to be compared with optical systems using refractive optical components, where it is difficult to find a material for the components which has good transparency over a wide spectral range, and such systems are generally achromatic i.e. their focal plane is the same for just a very narrow band of wavelengths.
The optical system is preferably light. Reflective optics offer significant weight saving over equivalent refractive, perhaps 20%, or 50% saving.
Each individual reflective optical component preferably has a high a transmission as possible, for example 0.95 or more, or 0.975 or more, or 0.985 or more, even 0.99 or more. A compounded system compounds up the losses and so if several optical elements are used it is desirable to use as many with high transmission as possible.
The optical system may further comprise an image plane fixed relative to the first and second variable focal length reflective optical components.
The optical components, which are preferably reflective, may also be used to stabilise an optical beam incident on the system. This can be used to reduce the problem of handshake when the optical system is used in a camera and/or reduce the effects of atmospheric aberration/compensate for other effects.
One or more of the reflective optical components may comprise a mirror. The control means may comprise a mechanical linkage, electronic circuit or a computer program. The or each reflective optical component may be deformed to vary its focal length.
Variation of the focal lengths of the reflective optical components is preferably carried out at high speeds, for example of the order of 10 Hz or more, 20 Hz or more, 40 Hz or more, or even 60 or 100 Hz or more.
One of the optical parameters controlled by the optical system may be the magnification of the system. The magnification achieved by the system may be variable by a factor of 3 or more, or 5 or more, or even 10 or more. The magnification may be variable between xc3x971 and, say, xc3x9710 or xc3x9715, or above.
Tilt means, such as mirror manipulation means, may be used to apply a tilt of at least one of the optical components about at least one axis. The tilt means may apply a two-axis-tilt to one or more of the reflective optical components, for example when magnification of the optical system is controlled. This may ensure that the optical axis of the system is maintained during magnification.
The optical system may be used in a magnification, or zoom, unit. The magnification unit may be used in conjunction with a camera.
Two or more optical systems may be used in conjunction with each other. One input may be supplied to such a xe2x80x98stackedxe2x80x99 optical system, the same input may be used to control both of the control means of the stacked system (or different control signals may be provided).
According to a sixth aspect of the present invention, there is provided an imaging system comprising an image detector and an optical system according to any preceding aspect of the present invention.
The imaging system may further comprise de-focusing means to de-focus one or more of the, preferably reflective, optical components. This de-focuses the image on the detector, which minimises reflection from the detector surface. When the imaging system is to be used with a laser, this may also reduce the laser energy density at the detector surface and hence reduces the risk of damage of the detector by the laser. The de-focusing means may be used to de-focus one or more of the, preferably reflective, optical components when the detector is not in use. Control means may ensure that for a substantial part (e.g. at least xc2xc or ⅓ of the time or at least xc2xd of the time, or at least xc2xe of the time) of the duty cycle of the detector (or detector array) the image incident upon the detector is significantly de-focused (e.g. a nominal focused spot may be de-focused to have an increase in area of 50% or more, 100% or more, 200% or more, 400% or more, 1,000% or more.
Means may be provided to detect an incident beam that is more intense than a predetermined threshold and automatically de-focus the optical system, at least for the pixel or pixels that would otherwise receive radiation above the threshold intensity. The entire image may be de-focused in response to a signal of too great an intensity.
It may be desirable to have none, or only one or two refractive components in the beam path (they are less transmissive than reflective components).
In order to minimise the risk of damage to a detector pixel in a detector array an incident image may be de-focused practically all of the time, without significant loss of image quality from the detector array if the degree of de-focus is controlled to match the system resolution. The output resolution of a pixellated array is controlled in part by the pixel size and geometryxe2x80x94there is little benefit in having input optical resolution better than the pixel output resolution. It is possible to take advantage of this by having the optics deliberately de-focus the image incident onto the detector array to the extent consistent with not degrading too much the output signals/picture of the array. FIG. 10 illustrates this. For any particular level of magnification the degree of desirable de-focus may be differentxe2x80x94at higher magnification less deliberate de-focus may be desirable as resolution may already be challenged.
According to another aspect the invention comprises an infra-red detecting device comprising an infra-red detector and an optical system which comprises at least a first and a second variable focal length optical components, preferably reflective components, whose positions are fixed relative to one another, and control means for varying the focal length of the first or second variable focal length optical component in relation to the focal length of the other of the said variable focal length optical components, such that, in use, control over at least two optical parameters of the optical system is achieved.
Preferably the device is an I.R imaging device, and it is preferably a thermal I.R. device. Thermal wavelengths may be considered to be about 3 to 14 microns.
The optical parameters controlled are preferably the focal lengths of the first and second optical components.
The thermal imaging system may further comprise reference means for providing a reference signal to the detector. De-focus means may be provided for controlling one or more of the optical components so as to cause the image of the scene received by the detector to be de-focused.
The de-focusing means may comprise the referencing means. If the scene image is substantially completely de-focused, the de-focused image of the scene may be indicative of the mean background temperature of the scene. It has long been desirable to produce a reference signal (periodically) that has a relationship with the captured scene, such as a signal indicative of the mean scene temperature, but hitherto the reference signal from mechanical choppers have typically been indicative of the camera temperature. Each pixel of a detector array of pixels will receive a mean scene temperature input if the scene image is fully de-focused (e.g. the input of the system is focused onto the detector array).
The infra-red imaging system may alternatively or additionally comprise dither means to manipulate the first or second optical component (or first and second optical component) between two configuration(s) so as to cause the detected image to dither between two positions on the detector. The dither means may change the shape of the optical component, e.g. mirrors, at a rate of 10 to 50 Hz, preferably 20 Hzxc2x110 Hz, or 30 Hzxc2x120 Hz. This allows two slightly differently focused images of the scene to be incident on the detector. This is conventionally achieved by using the two prism sections of a chopper device. This is A.C. coupling of the scene signal. It is important in some I.R. imaging systems to A.C. couple the detector signals to reduce noise.
It is also possible to control the shape of the two optical components so as to compensate for image aberration/camera shake/other effects without the need to have additional specific dedicated compensating opticsxe2x80x94i.e. the same optics that controls the magnification/zoom can be used to compensate for other effects. This can reduce weight and complexity.
There may be only two optical elements of variable focal length on the apparatus. There may be no other focusing optical elements. Alternatively, one or two (or more) additional optical components (reflective or refractive) may be provided for example to compensate for aberration and/or wide angle light collection (e.g. a Schmidt corrector).
According to another aspect, the invention comprises a method of providing A.C. coupling (or dither) in an electronic imaging detector comprising controlling a mirror (or lens) upon which radiation from the incident scene is incident so as to at one point in time direct an image to one position on an imaging detector, moving the mirror so as to direct the image at a second point in time to a different position on the imaging detector, and moving the mirror so as to repeatedly dither the image on the imaging detector, and in which the mirror (or lens) is distorted to achieve the effect rather than being bodily moved longitudinally or bodily rotated.
Preferably the mirror (or lens) is controlled so as to direct de-focused radiation from the scene onto the imaging detector. Preferably the de-focused radiation is substantially completely de-focused so as to present mean scene radiation to the detector. Preferably the mean scene radiation is periodically directed on the detector and is used as a reference in processing detected signals from the detector array.
The mirror may also be used to focus the image onto the imaging detector. The mirror may be part of zoom magnification optics and may be used at times to zoom the magnification of the image incident upon the imaging detector.
The method may also preferably comprise providing another focusing optical element in addition to the mirror (or lens) having the mirror, other focusing element and imaging detector fixed distances from each other, zoom being achieved by varying the focal length of the other optical element and the mirror (or lens).
The mirror may also be used to compensate for vibrations or movement. The other optical element is preferably bigger than the mirror and may be moved or perturbed less often, or at a slower rate, than is the smaller mirror. Radiation may be incident upon the other optical element before it is incident upon the mirror.
According to another aspect the invention comprises a method of providing a periodic reference signal in an electronic imaging detector comprising operating the detector in a mean scene mode from time to time in which a mirror (or lens) is controlled to de-focus completely, or substantially completely, radiation from a scene so that radiation incident upon the imaging detector is uniform, and using a signal produced by the detector during this mean scene radiation mode of operation as a mean scene reference signal, and controlling the mirror (or lens) to return to an imaging mode of operation after the reference has been obtained.
The detector preferably switches to mean scene reference mode with a regular periodicity, preferably of the order of many times per second.
According to another aspect, the invention comprises a method of minimising damage to detectors in an optical system and retro-reflection from an optical detector system, the method comprising operating optical focusing components of the system in a de-focus mode in which the captured radiation incident upon a detecting element is significantly defocused.